1. Field of the Invention
This invention relates to power transmission belts and, more particularly, to a method of forming side surfaces on belt ribs with a grinding element to avoid variation in the frictional coefficient between the side surfaces and cooperating pulley surfaces by reason of wear on the grinding element due to extended use thereof.
2. Background Art
V-ribbed belts are being used increasingly in different environments. V-ribbed belts are commonly utilized on automobiles, agricultural implements, and domestic electrical devices, to name just a few applications.
The V-ribbed belt operates differently than a V-belt and has certain advantages thereover. The V-ribbed power transmission belt is normally thinner than a V-belt. Resultingly, it is more flexible and can be wrapped around small diameter pulleys to contribute to an overall compact system design. Further, the ribs on the V-ribbed belt do not move as deeply into cooperating pulley grooves as do the ribs on a conventional V-belt. Consequently, there is less binding that occurs between the belt and pulley as the ribs move into and out of the cooperating pulley grooves, compared to a conventional V-belt. Energy losses through friction generation are minimized. The smooth transition as the belt moves into and out of a cooperating pulley results in a lengthening of the V-ribbed belt life compared to a V-belt.
The binding between the ribs and pulley with a conventional V-belt causes a nonuniform tension throughout the belt length. Further, this binding produces unwanted noise during operation.
V-ribbed belts do, however, have a number of drawbacks. Excessive tension on the V-ribbed belt may cause it to slip relative to a cooperating pulley. Further, abrupt changes in load or drive force may also cause the same slippage. This slippage, in addition to adversely affecting the operation of the system with which the belt is associated, may cause excessive wear, crack generation, and localized removal of material from the ribs by the pulleys.
It has always been an objective of designers of V-ribbed belts to reduce noise generation. A certain amount of noise is generated when slippage between the belt and pulley occurs as well as when the belt ribs move radially into and out of cooperating pulley grooves. This latter condition is a result of what is commonly referred to as "stick-slip" friction in a radial direction.
It is known to reduce noise generation by covering the ribbed surface with a cloth layer. This cloth layer not only reduces noise but enhances the wear characteristics of the rib rubber. The principal drawback with using a cloth layer is that the method for coating the rib rubber with cloth is relatively complicated. Another drawback is that the flexibility of the belt is deteriorated as is its bending fatigue resistance.
It is further known to mix short staple reinforcing fibers laterally in the ribs of a V-ribbed belt. These fibers, in addition to reinforcing the rib rubber, also tend to damp noise as the ribs impact and slide against cooperating pulley surfaces.
Typically, the belts with the short staple reinforcing fibers are manufactured by sequentially building belt components onto a forming drum/mandrel. The components are assembled in inside out order. A typical sequence of parts buildup is disclosed in Japanese patent publication No. Sho 52-17552. It is taught therein to apply a canvas layer, an upper rubber layer, tensile cords, and a lower rubber layer on a mandrel/forming drum. The assembled components are then vulcanized. The outermost rubber layer is formed using a rotary, stone grinding wheel. The grinding wheel has V-shaped cutting surfaces to produce grooves between adjacent V-ribbed belts or grooves between adjacent ribs on V-ribbed belts. The cutting surface of the grinding wheel forms the pulley-engaging side surfaces on the ribs.
While this method has been successfully practiced in the past, one problem that has been contended with is that due to the variation in the characteristics of the grinding wheel over time. Whereas a new grinding wheel tends to effect a clean severance of the fibers, wear on the grinding wheel over time inhibits the ability of the grinding wheel to cleanly sever the fibers and results in the fiber's protruding progressively further beyond the rib side surfaces as the grinding wheel wears in use. Since the frictional characteristics of the rib side surfaces depend upon the extent of exposure of the fibers, the belt performance varies significantly depending upon the age of the grinding wheel. Thus, quality control becomes difficult.
To avoid significant variation in the frictional characteristics of the belt, the grinding wheels may be periodically replaced. Commonly, the grinding wheels are made with a diamond polishing/cutting surface. These grinding wheels are relatively expensive.
Another problem that is encountered in the art results from the use of new grinding wheels. When the ribs of a V-ribbed belt are formed with a new grinding wheel, the reinforcing fibers are cleanly severed so that the ends thereof are flush with the rubber defining the belt side surface. During operation, the rubber coats the exposed ends of the fibers so that the friction reducing effect of the exposed fibers is progressively diminished. A large coefficient of friction results between the rib surfaces and pulleys to produce the aforementioned "stick-slip" condition. Eventually, the "stick-slip" problem may become excessive as the cut edges of the fibers become fully covered with rubber.
To eliminate this "stick-slip" problem, it is known to increase the quantity of short staple reinforcing fibers to reduce the coefficient of friction between the rib side surfaces and cooperating pulleys. However, the addition of fiber introduces other problems. First of all, as the proportion of fiber to rubber becomes greater, it is more difficult to uniformly mix the fibers without creating irregular disposition of the fiber. There is a tendency of the fibers to tangle and create voids which weaken the belt. Further, the additional fibers inherently compromise the bending characteristics of the belt.